HAL Id: hal-01894320https://hal.archives-ouvertes.fr/hal-01894320

Submitted on 12 Oct 2018

HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, estdestinée au dépôt et à la diffusion de documentsscientifiques de niveau recherche, publiés ou non,émanant des établissements d’enseignement et derecherche français ou étrangers, des laboratoirespublics ou privés.

Two Monopole Antennas for Generating Radio OAMWaves in Circular Waveguide

W. Wei, K. Mahdjoubi, C. Brousseau, O. Emile, A. Sharaiha

To cite this version:W. Wei, K. Mahdjoubi, C. Brousseau, O. Emile, A. Sharaiha. Two Monopole Antennas for GeneratingRadio OAM Waves in Circular Waveguide. EuCAP2016 - European Conference on Antennas andPropagation, Apr 2016, Davos, Switzerland. �hal-01894320�

Two Monopole Antennas for Generating Radio OAM

Waves in Circular Waveguide

W. L. Wei1, K. Mahdjoubi1, C. Brousseau1, O. Emile2, A. Sharaiha1 1 Institute of Electronics and Telecommunications of Rennes (IETR), University of Rennes 1, Rennes, France

2 Laser Physics Laboratory (LPL), University of Rennes 1, Rennes, France

kouroch.mahdjoubi@univ-rennes1.fr

Abstract—We present a novel way to generate waves bearing

Orbital Angular Momentum (OAM) in a circular waveguide by using two monopole antennas. The OAM mode l = 1 is generated by a combination of the classical TM01 and TE21 modes of the

circular waveguide. Two printed monopole antennas are used to create these two modes. The design procedure of this novel “OAM antenna” is presented. The resulting electrical field

distribution and the radiation and phase patterns evidence the properties of waves bearing OAM.

Index Terms—orbital angular momentum, circular

waveguide, monopole antenna.

I. INTRODUCTION Orbital angular momentum (OAM) has been proposed to

improve spectral efficiency [1–4] in radio communications, by

creating multiple sub-channels of propagation corresponding to

the twisting degree of the electromagnetic wave.

Whereas the phase of a usual plane wave is constant on the

wave front, the phase α of OAM waves undergoes a linear

variation along the angular coordinate (roll angle): α = l,

where l is an integer called the “topological charge” or the

order of the OAM mode.

Up to now, in radio frequency bands, two main families of

antennas have been proposed to generate OAM waves: circular

phased arrays [1, 5–7] and plane wave transformers (spiral

reflector, spiral phase plate and flat phase plate) [2, 8, 9].

In contrast, at optical frequencies different techniques are

used to generate OAM beams [10]. As an example, a helical

phased Laguerre-Gaussian mode LG01 (l = 1 or -1) is obtained

by combining two high-order Hermite-Gaussian modes of

HG01 and HG10 [11]. Along this way, a rectangular silicon

waveguide is already used to generate optical OAM beams by

combining the Ex

21 and Ex

12 modes [12].

In the same manner, by combining some proper modes of a

metallic waveguide we can create OAM waves in radio

frequency bands.

In this paper, we combine the classical TM01 and TE21

modes of a circular metallic waveguide to generate radio OAM

waves bearing a mode l = 1. Two printed monopole antennas

are introduced inside the circular waveguide to create the

desired modes of TE21 and TM01.

II. MODAL ANALYSIS

The electrical field distributions of the TM01 and TE21

modes are reminded in Fig. 1.

(a) (b)

Fig. 1. Electrical field distributions of the classical modes in a circular

waveguide: (a) TM01 mode; (b) TE21 mode

A. TM01 Mode

The magnitude and phase patterns of Ex and Ey components

for the TM01 mode are shown in Fig. 2. One can see that the

magnitude of the Ex component is minimal at the centre and in

this respect; it is similar to an OAM wave. In fact, the beam of

an OAM wave has a vortex with a minimum value of the field

intensity in the centre and a linear phase rotation against the

roll angle. For the TM01 mode, the phase of the Ex component

can be divided into two parts. For each part, the phase is

constant on the wave front, and the phase difference between

the two parts equals π. Due to the symmetrical distribution of

the electrical field, the Ey component has an identical

distribution compared to the Ex component, the only difference

is that both the magnitude and phase patterns rotate π/2.

(a)

(b)

Fig. 2. Magnitude (left) and phase (right) patterns of the TM01 mode: (a) Ex

component; (b) Ey component

B. TE21 Mode

The magnitude and phase patterns of Ex and Ey components

for the TE21 mode are shown in Fig. 3. We can observe that

there is quite a similar distribution compared to the TM01

mode. The main difference is that for each component, there is

a π/2 rotation of both magnitude and phase patterns.

(a)

(b)

Fig. 3. Magnitude (left) and phase (right) patterns of the TE21 mode: (a) Ex

component; (b) Ey component

C. Combination of TM01 and TE21 Modes

To create the OAM guided wave, we simultaneously excite

the TM01 and TE21 modes with a π/2 phase difference. The

resulting magnitude and phase patterns of Ex and Ey

components are shown in Fig. 4. We can observe that the

magnitude patterns have a null at the center and the phases

have a linear variation along the roll angle with a 2π phase

shift in one turn. This shows indeed that for each component,

both the amplitude and the phase correspond to a wave bearing

an OAM mode l equal to 1.

(a)

(b)

Fig. 4. Magnitude (left) and phase (right) patterns of the combined TM01 and

TE21 modes: (a) Ex component; (b) Ey component

III. ANTENNA DESIGN

A. Design Procedure

We introduce two printed monopole antennas inside a

circular metallic waveguide to generate the OAM waves. The

configuration of the proposed “OAM antenna” is shown in Fig.

5. The empty waveguide has a diameter of 4 cm and a height

of 5 cm, the corresponding cut-off frequencies of the TM01 and

TE21 modes are respectively 5.7 and 7.3 GHz. We choose a

working frequency of 8.3 GHz to allow the propagation of the

TM01 and TE21 modes and to prevent other high-order modes

existing.

Fig. 5. Configuration of the proposed OAM antenna

From the electrical field distributions of the TM01 and TE21

modes (see Fig. 1); we can find that to create these two modes,

the two monopole antennas should be placed on the same line

and be out of phase. To this purpose, the two monopole

antennas are connected with a single transmission line which is

back-fed at the centre point with a 50 Ω coaxial cable. In

addition, since the field intensity of the TE21 mode is maximal

at = ±45° and ±135° (see Fig. 1b), the monopole antennas

should also be placed at the corresponding positions. In our

design, we place the monopole antennas at = 45° and -135°.

The monopole antennas and the transmission line are both

manufactured on FR4 substrates with a thickness of 1.6 mm

and a relative permittivity of 4.4. The substrate totally has 4

parts. The 1st substrate is placed at the bottom of the

waveguide with the same diameter and the 2nd one, in the

middle of the waveguide. The 3rd and 4th substrates are placed

along the side wall of the waveguide and are used to connect

the 1st and 2nd substrates. The transmission line has a width of

0.7 mm and an impedance of 100 Ω. To satisfy the working

frequency and the impedance matching, the length and width

of each monopole antenna are optimized at 12 and 4.5 mm,

respectively.

B. Simulation Results

As depicted in Fig. 6, the antenna is well matched around

8.3 GHz.

The magnitude and phase patterns of the generated wave,

observed on a plane perpendicular to the direction of

propagation, are shown in Fig. 7. The observation window is a

circular area with a radius of 3 cm, lying 1 cm above the

waveguide aperture. It can be seen that for both Ex and Ey

components, the magnitude at the centre is much smaller than

the surroundings and the phase rotates around the centre with a

2π phase shift in one turn. This shows indeed that for each

component, both the amplitude and the phase of the generated

wave correspond to an OAM bearing wave with l = 1.

At a working frequency of 8.3 GHz, the TE11, TM01 and

TE21 modes are able to propagate in the waveguide. However,

the monopole antennas are out of phase and due to this fact,

the TE11 mode is eliminated. Thus, we can conclude that the

OAM wave is generated by a combination of the TM01 and

TE21 modes.

7.5 8 8.5 9 9.5-40

-30

-20

-10

0

Frequency(GHz)

Refl

ecti

on

Co

eff

icie

nt(

dB

)

Fig. 6. Reflection coefficient of the OAM antenna

(a)

(b)

Fig. 7. Magnitude (left) and phase (right) patterns of the generated wave: (a)

Ex component; (b) Ey component

Fig. 8 shows the 3D radiation and phase patterns of the

OAM antenna. For simplicity, we just present the results of the

Ex component. The radiation patterns in magnitude and phase

depict the vortex that is characteristic of an OAM wave. The

spiral form of the phase pattern, plotted in polar coordinates,

corresponding to the linear variation of the phase against the

roll angle (φ, in Fig. 8b).

(a)

(b)

Fig. 8. 3D radiation and phase patterns of the OAM antenna: (a) magnitude;

(b) phase

IV. CONCLUSIONS

In this paper, we have presented a method to generate radio

OAM waves in a circular metallic waveguide by a combination

of TM01 and TE21 modes. An antenna using two printed

monopoles and a single transmission line is designed to create

these two modes and hence, to generate an OAM wave with a

topological charge l = 1. Both the 2D magnitude & phase

patterns of the electrical field and the 3D radiation & phase

patterns confirm the generation of an OAM bearing wave.

ACKNOWLEDGMENT

The authors would like to thank China Scholarship Council

(CSC) (No. 201306090101) for scholarship support.

REFERENCES

[1] S.M. Mohammadi, L. Daldorff, J. Bergman, R. Karlsson, B. Thidé, K. Forozesh, T. Carozzi, and B. Isham, “Orbital angular momentum in radio – a system study,” IEEE Trans. Ant. Propag., vol. 58, no. 2, pp. 565–572, 2010.

[2] F. Tamburini, E. Mari, A. Sponselli, B. Thidé, A. Bianchini, and F. Romanato, “Encoding many channels on the same frequency through radio vorticity: first experimental test,” New. J. Phys., vol. 14, pp. 033001, 2012.

[3] J. Wang, J.Y. Yang, N. Ahmed, S. Dolinar, and A.E. Willner, “Terabit free-space data transmission employing orbital angular momentum multiplexing,” Nat. Photon., vol. 6, pp. 488-496, 2012.

[4] Y. Yan, G.D. Xie, N. Ahmed, M. Tur, and A.E. Willner, “High-capacity millimetre-wave communications with orbital angular momentum multiplexing,” Nat. Commun., vol. 5, 4876, 2014.

[5] B. Thidé, H. Then, J. Sjöholm, K. Palmer, J. Bergman, T.D. Carozzi, Ya.N. Istomin, N.H. Ibragimov, and R. Khamitova, “Utilization of photon orbital angular momentum in the low-frequency radio domain,” Phys. Rev. Lett., vol. 99, pp. 087701, 2007.

[6] Q. Bai, A. Tennant, and B. Allen, “Experimental circular phased array for generating OAM radio beams,” IET Electron. Lett., vol. 50, no. 20, pp. 1414–1415, 2014.

[7] W.L. Wei, K. Mahdjoubi, C. Brousseau, and O. Emile, “Generation of OAM waves with circular phase shifter and array of patch antennas,” IET Electron. Lett., vol. 51, no. 6, pp. 442–443, 2015.

[8] G.A. Turnbull, D.A. Robertson, G.M. Smith, L. Allen, and M.J. Padgett, “The generation of free-space Laguerre-Gaussian modes at millimeter-wave frequencies by use of a spiral phaseplate,” Opt. Commun., vol. 127, pp. 183–188, 1996.

[9] R. Niemiec, C. Brousseau, K. Mahdjoubi, O. Emile, and A. Ménard, “Characterization of an OAM flat plate antenna in the millimeter frequency band,” IEEE Antennas Wirel. Propag. Lett., vol. 13, pp. 1011–1014, 2014.

[10] M. Padgett, and L. Allen, “Light with a twist in its tail,” Contemp. Phys., vol. 41, no. 5, pp. 275-285, 2000.

[11] L. Allen, M. Beijersbergen, R. Spreeuw, and J. Woerdman, “Orbital angular momentum of light and the transformation of Laguerre–Gaussian laser modes,” Phys. Rev. A, vol. 45, no. 11, pp. 8185-8189, 1992.

[12] D. Zhang, X. Feng, K. Cui, F. Liu, and Y. Huang, “Generating in-plane optical orbital angular momentum beams with silicon waveguides,” IEEE Photon. J., vol. 5, no. 2, pp. 2201206, 2013.